Improperly or incompletely maintained water distribution systems containing metal, plastic, concrete or concrete/asbestos pipe may show scale formation, sedimentation and microbiological tubercular growth by iron, manganese, sulfate-reducing, organic acid-producing, aerobic and other bacteria. This scale, sedimentation and growth may result in restricted water flow, higher pumping costs, customer complaints of the water's appearance, odor or taste, low chlorine residues, health hazards, system leakage and poor performance of the distribution systems.
Mechanical cleaning methods such as pigging, scraping, reaming and honing have been used to remove blockages from water distribution systems. These methods, however, require extensive excavation and opening of the distribution system for insertion of the appropriate tools. Valves must usually be removed and replaced along with hydrants, while elbows and hydrant connects are not usually cleaned mechanically and thus remain uncleaned. Fire protection systems such as fire sprinkler systems are impossible to clean mechanically.
Underscale corrosion causes small pits in the walls of systems which cannot be completely cleaned by mechanical methods. The residues cause immediate "red water" problems when the system is put back into service due to rust. In addition, residual bacterial growth results in new tuberculation with resulting reduced flow. Because of these residues, mechanical cleaning is normally followed by cement lining, epoxy lining, or other insertion/lining process. However, lining only covers up these residues. In addition, it decreases the diameter of the pipe and adds substantially to the rehabilitation cost.
Many of these blocked distribution systems can be cleaned by a low cost process using chemical cleaning solutions that are circulated in isolated sections of the system. One such method is disclosed in U.S. Pat. No. 5,360,488 which is assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety, along with assignee's U.S. Pat. No.5,527,395 covering a chemical cleaning process improvement, and co-pending U.S. Pat. No. 5,680,877 and U.S. patent application Ser. No. 08/675,802.
However, each distribution system's requirements for cleaning and passivating must be considered individually. Factors to consider in formulating a proper cleaning and passivating program include the source of the water, prior water treatment, water quality in terms of its pH, hardness and metal content, as well as economic factors. For example, in many chemically cleaned distribution systems the interior of the pipe is cleaned down to the bare metal, which is usually iron. Depending upon the water quality, pH, dissolved oxygen content and the like, the cleaned iron surface can form red iron oxide or hydroxide or corrosion products and may be the cause of a recurrence of red water. Specific factors, such as ensuring that treatment of potable water systems use only those corrosion and scale control agents which have been tested and certified to ANSI/NSF Standard 60, must also be considered.
In beginning a conventional potable water passivating program, a relatively higher level of passivating agent, in the range of approximately ten to thirty ppm, is added directly to water at the treatment plant. It may then take from several weeks to several months for the passivation layer to form throughout the entire distribution system. In many cases flushing is also required to establish the passivation layer, particularly in low flow or dead ends of the distribution system. Once the distribution system has been passivated, a lower concentration of passivating agent, in the range of approximately one to two ppm, must be continuously employed to maintain the passivating layer. Biocides may also be employed in water systems after cleaning.
In fire sprinkler systems different end use requirements are required due to the static nature of the water in the system which allows for microbiological growth and subsequent problems associated with the growth.
Therefore, a simple and effective method for chemically cleaning and then rapidly passivating and maintaining the chemically cleaned interior surface of various types of water distribution systems is needed.